Retardation films are optical members to be used for obtaining various kinds of polarized light such as linearly polarized light, circularly polarized light, and elliptically polarized light. As such retardation films, a λ/4 plate whose retardation is ¼ of a wavelength λ and a λ/2 plate whose retardation is ½ of a wavelength λ are known. The λ/4 plate has an optical function of converting linearly polarized light to circularly polarized light, and the λ/2 plate has an optical function of rotating the plane of polarization of linearly polarized light by 90°. Generally, such retardation films designed to act as a λ/4 plate or a λ/2 plate for light having a specific wavelength cannot perform the same function for light having any other wavelength. For example, a retardation film designed to act as a λ/4 plate for light having a wavelength of 550 nm cannot perform the same function for light having a wavelength of 450 nm or 650 nm. That is, retardation films generally exhibit wavelength dispersion characteristics such that the retardation thereof depends on wavelength. For example, it is generally known that a polymer film exhibits wavelength dispersion characteristics such that the retardation thereof is larger at a shorter wavelength and is smaller at a longer wavelength.
When white light being a composite of various wavelengths of visible light rays enters a retardation film exhibiting such wavelength dispersion characteristics, the wavelength dispersion characteristics cause a problem that the form of polarization of light rays greatly varies depending on their respective wavelengths and therefore distribution of polarization state occurs, so that the incident white light is converted to colored light.
In order to overcome such a problem, Japanese Patent Laid-open No. Hei 10-239518 has proposed a retardation film having a wavelength dispersion value α of less than 1, which is obtained by laminating together two or more birefringent media having different wavelength dispersion values α(α=Δn(450 nm)/Δn(650 nm)) in such a manner that their slow axes intersect at right angles. Japanese Patent Laid-open No. Hei 10-239518 describes that such a retardation film has the effect of giving a constant optical retardation to any wavelength lying within a visible light wavelength range (that is, such a retardation film provides a constant optical retardation irrespective of wavelength) so that white light can be easily obtained.
However, since the retardation film disclosed in Japanese Patent Laid-open No. Hei 10-239518 is a laminated product comprising two or more birefringent media, it requires a process for laminating these birefringent media and bonding them together. In addition to that, it is also necessary to select two or more birefringent materials and an adhesive for bonding two or more birefringent media. Further, since such a retardation film having a laminate structure becomes relatively thick, it is not suitable for use in, for example, liquid crystal displays required to be smaller in thickness.
It is therefore an object of the present invention to provide a retardation film which can be relatively small in thickness and which exhibits wavelength dispersion characteristics such that retardation for light having a wavelength of λ lying within almost the entire visible light wavelength range from 400 to 700 nm is about λ/2 or λ/4, a novel polymer that can be suitably used as a material for forming such a retardation film, an optical film, and an image display device.